Many electrical loads can be adversely effected if their supply of electrical power is disrupted for even a fraction of a second. Uninterruptible Power Supply (UPS) systems are commonly used to prevent such disruption when the normal supply of electrical power fails or falters. These UPS systems use a temporary energy source, such as batteries or flywheels, to provide power to their protected load for a limited ride through time of from several seconds to several minutes. Some UPS systems are coordinated with a standby engine-generator set, whereby the gen-set is automatically started in the event that the failure of the incoming utility power line is not quickly restored. Typically, the engine-generator set is electrically connected on the utility side (or normal power supply side) of the UPS system, and a transfer switch is used to connect it to the input of the UPS system once the engine is started and up to normal operating speed.
Traditional battery-based static UPS systems, or even flywheel-based rotary UPS systems which use a motor-generator set, completely regenerate their input power in order to protect their loads. Therefore, if the engine-generator set is intended to supply power to the protected load through the UPS system, then it must be sized to provide for the additional parasitic losses of the UPS system that is in series with the protected load. Also, this combination of engine-generator set and UPS system will have a reduced ability to provide short circuit currents needed to isolate faults properly or to regulate voltage during abrupt changes to the protected load (such as from large motor starting or load switching). Control of this combination of gen-set and UPS system is, however, simple. For example, the UPS system has its own utility power disturbance analyzer, and is able to tell when utility power has failed. After a set time delay, it can issue a start command to the engine-generator set to begin its standby operation. The engine-generator set has its own automatic control scheme to start the engine, bring the engine up to normal operating speed, close the transfer switch, and thereafter adjust its own power frequency using a governor to control engine throttle. The engine-generator set governor and the UPS frequency control circuits can work independently. For example, if there is a step change in the protected load, the UPS system will automatically try to maintain load frequency regardless of its input frequency from the gen-set. The governor of the gen-set will respond to the change in electrical load requirement it sees from the UPS system by automatically increasing the engine throttle setting when the engine slows down due to the higher torque required. This is a stable control system.
There are three problems associated with the above combination of engine-generator set and traditional UPS system. First, the normal parasitic losses of series in-line UPS systems are inherently high; typically 90 to 92% efficiency. This equates to high operating cost over the life of the equipment. Second is that the standby engine-generator set must be oversized to account for this low operating efficiency. This equates to higher initial capital costs for the gen-set, plus higher operating costs while on standby. Third, the reduced ability to supply short circuit current for proper fault clearing, and to regulate voltage during abrupt load changes creates an application problem for such systems. This last issue is one of the reasons why traditional UPS systems are not normally used to protect industrial process circuits, where loads are constantly changing and large motors are often switched on and off.
Integrated rotary UPS and engine-generator systems are available to address some of these problems. Such systems include a synchronous motor generator which can be driven by a temporary energy source (such as a flywheel) for a short duration, or an integrally-mounted engine for long duration. In normal operation, the synchronous motor-generator (SMG) is connected to the utility line as a lightly loaded synchronous motor. When a utility disturbance is detected, a circuit breaker automatically opens to isolate both the SMG and the protected load from the utility supply. The temporary energy supply then mechanically drives the SMG so that it becomes a generator to supply power to the protected load. At the same time, the engine is automatically started, brought up to speed, and a clutch engaged to allow the engine to mechanically drive the SMG after the temporary energy source is depleted. Such an integrated systems is termed a Continuous Power Supply (CPS) system, as it is able to continue to supply power to the protected load long after the temporary energy source of a traditional UPS is exhausted.
These CPS systems address some of the problems inherent in the traditional UPS/engine-generator set combination, but add other disadvantages. First, the efficiency of the system is improved over the series in-line type of UPS system (typically 92 to 94%), but still represents a significant operating loss over the life of the equipment. For example, for each percentage point of inefficiency at a utility rate of 5 cents per kilowatt-hour, a 1000 kW machine will cost $35,000 to operate over ten years. Second, the engine and the SMG do not have to be oversized because, during standby, the protected load is supplied directly from the SMG. While the CPS has slightly higher short circuit capability, and therefore slightly better voltage regulation during abrupt load changes, it still does not have the full capability of a typical utility supply. Further, it has several limitations that often make its application impractical. One of these is that it is not practical to retrofit a CPS where there is an existing engine-generator set. The generator cannot be used, there may not be sufficient space to accommodate the larger CPS assembly, and reconnection of the existing engine is expensive. Another problem is lack of flexibility in applying the "UPS" function versus the "engine-generator set" function. For example, it is often desirable to protect only part of a facility's electrical load with a "UPS" function, but connect a much larger portion of the facility to the emergency standby power of an engine-generator set. These application problems are inherent in the integrated construction of the traditional CPS system.